WASHINGTON — A day after claiming the title of Republican presidential nominee, Mitt Romney moved aggressively Wednesday to coordinate with the Republican National Committee to intensify his fight against President Barack Obama. One-time bitter GOP rivals looked to be coalescing behind the former Massachusetts governor.

RNC Chairman Reince Priebus marked the transition Wednesday by proclaiming Romney the party's "presumptive nominee." Romney's campaign also appointed several senior staff members to work on an informal takeover of the committee's national infrastructure.

"We will ensure that our finance, political and communications teams are fully synchronized," Priebus said. "I am excited that these two top-notch operations will start to integrate and present a unified team to defeat Barack Obama."

At the same time, fading Republican contender Newt Gingrich signaled that he would likely follow Rick Santorum out of the race and called on the GOP to unite behind Romney. Aides confirmed that Gingrich will leave the race next week and said he was likely to endorse his one-time rival.

The dramatically shifting landscape comes as Romney refocuses his efforts on challenging Obama, raising money for the battle ahead and reconciling with a divided Republican Party.

"Tonight is the start of a new campaign," Romney said Tuesday night as he celebrated a sweep of five primaries. He blasted Obama as a man whose tenure has been marked by "false promises and weak leadership" in a time of economic struggle.

The contests were the first since Santorum conceded the race, and the former Pennsylvania senator said he intended to sit down with Romney's representatives on Wednesday and with Romney himself in the next week or two.

"Mitt Romney is going to be the nominee," Santorum told CNN, "and I'm going to support the nominee."

Romney was attending fundraisers Wednesday and Thursday to prepare for what may be the most expensive presidential contest in the history of American politics. He exuded confidence Tuesday night, but faces a 10-to-1 cash disadvantage in a general election matchup against the Democratic president.

Romney has at least six closed-door fundraisers in two days in New York and New Jersey. They may be among his final private meetings with donors, according to campaign officials who confirmed that Romney would begin opening some finance events to reporters as early as next week. The officials requested anonymity to discuss internal decisions.

The move would follow an embarrassing episode in which reporters outside a closed fundraiser in Palm Beach, Fla., overheard Romney sharing previously undisclosed details about his tax plan. Romney has faced growing calls for transparency in his role as the GOP's likely candidate.

One campaign official said Romney would probably begin inviting a small group of reporters into larger fundraisers, particularly those in which he makes remarks. That's largely the policy Obama follows.

While Romney essentially declared the beginning of the general election Tuesday night, he has been free to focus on Obama since Santorum suspended his campaign two weeks ago. That ended a nasty primary battle that took a heavy financial toll and prevented Romney from stockpiling cash to use against his Democratic opponent.

The process of formally integrating his campaign with the RNC had already been under way.

Three Romney advisers spent two days at an annual state party gathering in Arizona last week to start laying the groundwork for the effort. Longtime Romney confidante Ron Kaufman, also an RNC member, organized that effort and will continue to serve in such a role.

Republican operative Brian Jones, a veteran of John McCain's 2008 presidential campaign, will take the lead on coordinating between the two entities. Kevin Madden, who was Romney's spokesman in 2008, will advise the communications team.

Gingrich and Santorum aggressively questioned his conservative credentials during the primaries with Santorum at one point declaring Romney the worst candidate to face Obama. But asked Tuesday night on CNN if Romney was "the right guy" to represent the Republican Party, Santorum said he was.

Romney's success will depend, at least in part, on his ability to compete financially with Obama.

The Republican's campaign had about $10 million in the bank at the end of March, according to federal filings. Obama reported having more than $104 million in his account; he'd already spent $90 million on the general election.

Romney was eager to turn the political page after Tuesday's primary wins in New York, Pennsylvania, Connecticut, Rhode Island and Delaware.

"After 43 primaries and caucuses, many long days and not a few long nights, I can say with confidence – and gratitude – that you have given me a great honor and solemn responsibility," he told supporters gathered in New Hampshire. He urged all who are struggling in a shaky economy to "hold on a little longer – a better America begins tonight."

Romney is only 300 delegates away from the 1,144 delegates it takes to formally secure his party's nomination, and that could happen by the end of May.

Obama set the modern fundraising record in 2008, when he and his Republican rival, Sen. John McCain, spent more than $1 billion combined; Obama spent more than $730 million.

The two major-party candidates set a spending record of $700 million in 2004.

Obama opened his finance events to press coverage in June 2008 after he became the presumptive Democratic nominee and largely plays by the same rules as president. Reporters are allowed in if he makes remarks during the event, regardless of size, and are kept out if there are no formal remarks.

Romney's campaign has refused to provide specific times and locations of this week's fundraisers.